So I found a dowel rod online that’s 1 meter long by 25 mm in diameter made of beech, which is pretty typical for this kind of rod. Each rod weighs 420 g. 300,000 km is 300,000,000 m. So for a dowel rod to be 300,000,000 m long, it would weigh 126,000,000,000 g, or 126,000,000 kg. You would never be able to push this rod. If you had a magical hydraulic ram that could, it would just compress the soil under it. This is on the scale of the foce released from an atomic bomb.
But let’s throw that out and pretend the whole thing weighs 420 grams instead. Maybe it’s made of a novel, space-age material instead of beech. And since you’ve said it can’t bend or break, the portion at the surface of the earth would be spinning at roughly 1,000 kph (due to the rotation of the earth), and the portion at the end of the rod would be spinning at about 28 km/s. Most of the mass of the rod would be spinning faster than escape velocity, so you wouldn’t be able to hold onto it. It would be gone almost instantly.
Let’s pretend you could hold onto it. Then the person on the moon couldn’t hold it, because the earth rotates on its axis about 28 times faster than the moon travels around its orbit. So you can see how this problem devolves into ever more layers of magic and hand-waiving.
The final problem is the fundamental difference between classroom physics and material engineering. If you could fix the moon to the end of the rod, and you used a space-age material that weighs 420 g for the whole thing, and it could be so rigid as to not bend, then it would have to break instead. If, instead, it’s designed to not break, then it must be able to bend. This is just how real materials work. But even if it does neither, or at most only bends a little, it is still true that as you push on the rod it would compress. So the tip wouldn’t move at first. The pressure would move through the rod like a wave. You can’t send information faster than light.
Yes, about my setting, it was pretty much an excuse to illustrate the experiment, with like you said, a bit too much of magic.
The moon being on a straight distance of approximately 1 light second, i didn’t had found another place to put this experiment on. So I didn’t take into account the herculean strengh needed, the movement of the earth and the moon and the gravity.
Someone gave a link to an answer of my question, with a more realistic take on the position of the other end, but your explanations are still welcome for this moon setting and the “moon elevator” problem :)
(i know i may have broken english sometimes, sorry about that)
(i know i may have broken english sometimes, sorry about that)
Not at all! I couldn’t tell you aren’t a native speaker. Regarding a “moon elevator”, or more realistically a space elevator, these kinds of Herculean physics problems are exactly what people are trying to iron out. The forces involved are astronomical.
Yeah IIRC that even applies to things like gravity as well. As in, we aren’t actually orbiting around where is sun is, we’re orbiting around where it was ~8 minutes ago because the sun is about 8 light-minutes from Earth.
No, gravity is faster than light. If there was this lag, we wouldn’t have stable orbits exactly because of the lag you describe. Wave functions of photons also collapse faster than light when they hit absorbent material.
I don’t think gravitational waves traveling at the speed of light is the same as the gravitational attraction being apparently felt faster than light travels. Similarly, electric attraction between + and - charges is different from electromagnetic waves being transmitted in the field. It’s not light that is “communicating” that attraction.
I don’t think gravitational waves traveling at the speed of light is the same as the gravitational attraction being apparently felt faster than light travels.
I don’t know how you would measure gravitational waves without measuring gravitational attraction.
It’s not light that is “communicating” that attraction.
Just to clarify: when people talk about the speed of gravity, they mean the speed at which changes propagate. It’s the answer to questions like: if I take the Sun and wiggle it around, how long does it take for the Earth to feel the varitation in the force of gravity? And the answer is that changes in gravity travel at the speed of light.
But that’s not what you’re asking about. Whenever you’re close to the Earth, gravity is always acting on you: it’s not waiting until you step off a cliff, like in the Coyote and the Roadrunner. The very instant your foot is no longer on the ground, gravity will start to move it downwards. The only detail is that it takes some time for it to build up an appreciable speed, and this is what allows us to do stuff like jump over pits: if you’re fast enough, gravity won’t be able to accelerate you enough - but gravity is still there.
I get the sense that you’re thinking about the second scenario when objecting to the concept that gravity travels at the speed of light.
So I found a dowel rod online that’s 1 meter long by 25 mm in diameter made of beech, which is pretty typical for this kind of rod. Each rod weighs 420 g. 300,000 km is 300,000,000 m. So for a dowel rod to be 300,000,000 m long, it would weigh 126,000,000,000 g, or 126,000,000 kg. You would never be able to push this rod. If you had a magical hydraulic ram that could, it would just compress the soil under it. This is on the scale of the foce released from an atomic bomb.
But let’s throw that out and pretend the whole thing weighs 420 grams instead. Maybe it’s made of a novel, space-age material instead of beech. And since you’ve said it can’t bend or break, the portion at the surface of the earth would be spinning at roughly 1,000 kph (due to the rotation of the earth), and the portion at the end of the rod would be spinning at about 28 km/s. Most of the mass of the rod would be spinning faster than escape velocity, so you wouldn’t be able to hold onto it. It would be gone almost instantly.
Let’s pretend you could hold onto it. Then the person on the moon couldn’t hold it, because the earth rotates on its axis about 28 times faster than the moon travels around its orbit. So you can see how this problem devolves into ever more layers of magic and hand-waiving.
The final problem is the fundamental difference between classroom physics and material engineering. If you could fix the moon to the end of the rod, and you used a space-age material that weighs 420 g for the whole thing, and it could be so rigid as to not bend, then it would have to break instead. If, instead, it’s designed to not break, then it must be able to bend. This is just how real materials work. But even if it does neither, or at most only bends a little, it is still true that as you push on the rod it would compress. So the tip wouldn’t move at first. The pressure would move through the rod like a wave. You can’t send information faster than light.
That was excellent. Thank you
Yes, about my setting, it was pretty much an excuse to illustrate the experiment, with like you said, a bit too much of magic.
The moon being on a straight distance of approximately 1 light second, i didn’t had found another place to put this experiment on. So I didn’t take into account the herculean strengh needed, the movement of the earth and the moon and the gravity.
Someone gave a link to an answer of my question, with a more realistic take on the position of the other end, but your explanations are still welcome for this moon setting and the “moon elevator” problem :)
(i know i may have broken english sometimes, sorry about that)
Not at all! I couldn’t tell you aren’t a native speaker. Regarding a “moon elevator”, or more realistically a space elevator, these kinds of Herculean physics problems are exactly what people are trying to iron out. The forces involved are astronomical.
Yeah IIRC that even applies to things like gravity as well. As in, we aren’t actually orbiting around where is sun is, we’re orbiting around where it was ~8 minutes ago because the sun is about 8 light-minutes from Earth.
No, gravity is faster than light. If there was this lag, we wouldn’t have stable orbits exactly because of the lag you describe. Wave functions of photons also collapse faster than light when they hit absorbent material.
wave function (something that does not travel) collapses (something that does not move either) faster than light (themselves?)
this word soup does not make sense
https://bigthink.com/hard-science/speed-of-gravity/
I don’t think gravitational waves traveling at the speed of light is the same as the gravitational attraction being apparently felt faster than light travels. Similarly, electric attraction between + and - charges is different from electromagnetic waves being transmitted in the field. It’s not light that is “communicating” that attraction.
I don’t know how you would measure gravitational waves without measuring gravitational attraction.
Nobody said it was. The “speed of light” isn’t about “light”. Gravity propagates at the same speed, aka “c.”
This Reddit discussion on r/AskPhysics might help clear up your misconceptions. Notably:
I get the sense that you’re thinking about the second scenario when objecting to the concept that gravity travels at the speed of light.